1. Technical Field
The present disclosure relates stereo recording and more specifically to acoustic echo control in stereo recording.
2. Introduction
Traditional stereophonic capture of audio results in two highly correlated signals. FIG. 2 illustrates an example of different microphone types. An omnidirectional microphone 202 has a generally sphere-shaped sensitivity pattern. A cardioid microphone 204 is a form of unidirectional microphone with a generally heart-shaped sensitivity pattern. FIG. 2 also illustrates a third type of unidirectional microphone 206. The prior art approaches below are discussed in terms of these types of microphones.
The A/B technique 300 or time-of-arrival telephone approach, as shown in FIG. 3, captures stereophonic audio via a left omnidirectional microphone 302 and a right omnidirectional microphone 304 spaced far apart 306 one from another. An example distance is 50 centimeters. This approach has the advantage of using less expensive omnidirectional microphones, but the spacing requirements (i.e. tens of centimeters or more) render this stereophonic approach impractical for most telephony applications, especially in mobile devices where smaller is better.
The X/Y technique 400 or intensity stereophony approach, as shown in FIG. 4, captures stereophonic audio via left cardioid microphone 402 and a right cardioid microphone 404. Under this approach, true coincidence is quite hard to obtain in telephone housings. Further, cardioid microphones are several times more expensive than omnidirectional microphones.
In addition, these stereophonic approaches often introduce echoes that should be cancelled out to provide a more clean sound when sound is transmitted from one endpoint to another in a full-duplex fashion. Stereophonic acoustic echo cancellation requires a pre-processing step to decorrelate the loudspeaker signals in order for the adaptive filters to converge to a good solution. One way to decorrelate the signals is to non-linearly modify the signals or to apply time-varying all-pass filters. However, these pre-processing steps lead to audible distortion of the stereophonic signals and/or destruction of the spatial image originally captured by the stereophonic recording. Even state-of-the-art approaches distort the loudspeaker signals and/or destroy the original spatial image of the recorded acoustic space.